Banks of fluid-carrying tubes often experience flow-induced vibrations when subjected to external fluid crossflow in tube-in-shell heat exchangers, nuclear fuel bundles, steam generators and the like. The excitation mechanisms typically involved are vortex shedding, turbulence and fluidelastic instability. The first two vibrations are generally present throughout the anticipated load range, but the tube structure is typically designed to be capable of withstanding their long-term effects. However, fluidelastic type vibration is characterized by large amplitude displacements and often has a structurally damaging effect. A heat exchanger must, therefore, be designed to eliminate or minimize the damaging effects of fluidelastic vibration.
To reduce the impact of fluidelastic instability within the operating flow range, it is preferable to design the tube bank so that the critical crossflow fluidelastic velocities are sufficiently greater than the maximum design crossflow velocity. A minimum safety factor of 1.5, representing the ratio between the critical and the maximum crossflow velocities, is typically sought to assure the absence of fluidelastic vibration. In order to obtain the necessary critical velocities, sufficiently high tube natural frequencies are needed. The required tube frequencies are typically achieved by shortening tube spans between adjacent supports or tube support plates and/or by providing reasonably close tolerances in tube-to-support clearances.
In the cases of flexible tube banks or banks exposed to high flow velocities, a large number of supports or support plates may be needed in order to maintain the desired ratio between the critical and actual flow velocities. This can increase costs, increase pressure drops, complicate erosion/corrosion considerations, complicate overall design, and undermine performance. Thus, it is desirable to utilize simpler designs with a minimum of main supports. This reduction in the number of supports may require the addition of tube-to-tube ties to achieve the required vibration resistance of the tube bank.
Many attempts have been made to address these vibration concerns by means of tube-to-tube ties. For example, U.S. Pat. No. 5,213,155, to Hahn, attempts to dampen vibrations resulting from temperature changes and fluid flow through and outside the tubes. The tubes 15, 19, 23 are clamped via metal tie fasteners 30, 40 between parallel strips 2, 3 of a U-shaped stake. Each strip portion 2, 3 has a "soft V" cross section and a plurality of longitudinally-spaced "saddles" 14, 16, 18, 20, 22, 24 that engage the tubes.
Another example, U.S. Pat. No. 5,136,985, to Krowech, is an attempt to increase mechanical stability and decrease vibrations due to gas flow around tubes in a heat exchanger. A plurality of the tubes 16 are interconnected by a support 20. A tie bar 30, with a plurality of spaced apart fingers 32, is positioned so that one boiler tube 16 fits between each pair of adjacent fingers 32. A locking bar 44 is wedged between the row of tubes 16 and a series of retainer pins 40 of the tie bar, clamping the tie bar to the tubes. The locking bar is then welded to either the tie bar or one of the tubes.
U.S. Pat. No. 3,708,142, to Small, shows that in order to suppress vibration of tubes 2 arranged in rows and columns in, for example, a tube-in-shell heat exchanger, the tubes are held together in three-dimensional bundles. Support rods 16, 26 extend across the rows and columns, respectively, of tubes, with one rod positioned between each tube row or column. Each successive rod is laterally spaced a common distance along the length of the tubes. Securing means 36, 38, such as metal bands, are attached to the ends of each rod to maintain and urge the rods together to form a unitary tube bundle.
U.S. Pat. No. 4,550,777, to Fournier, et al., shows an attempt to reduce stresses from weight distribution, vibrations, and thermal expansion in tubes in a heat exchanger, in which the tubes are suspended vertically in a serpentine arrangement from inlet and outlet ends. Lengths of the tube are interlocked to transfer the weight of the middle tubes to the outer tubes depending from the inlet and outlet. Complementary interlocking members 12, 13 are welded to adjacent tubes and interlocked as shown in FIG. 2. A stop 22 is welded to one of the tubes to prevent the interlocking members from disengaging.
U.S. Pat. No. 3,929,189, to Lecon, shows another attempt to reduce the vibration of tubes in a heat exchanger. A structural framework is formed of a plurality of flat bars 66 interconnected at their respective upper and lower ends by tie bars 68. Adjacent bars 66 are drawn tightly against a row of the tubes and interconnected by second retaining members 72. The framework preserves the spacing between the tubes and prevents direct contact between the tubes.
While the known designs may help to control the intertube motion in the tube banks, most do so by tightly locking the tubes in a bundle. Thus, these approaches do not provide for maintaining a reasonably flexible tube bundle. These limitations can produce undesirable restrictions on the thermal expansion of the individual tubes. They can also create a rigid multiple-tube structure that has additional, inherent vibration concerns itself.